Recent years, an organic electroluminescence element (hereinafter referred to simply and occasionally as “organic EL element”) in which a voltage is applied to a luminescent layer provided between electrodes to cause the luminescent layer to emit light has been actively researched and developed. Because of having excellent properties: luminous efficacy, low-voltage drivability, lightweight and low cost, the organic EL element is utilized as a planar type illumination, a light source for an optic fiber, a backlight for a liquid crystal display, a backlight for a liquid crystal projector, and various other light sources for a display device and others, and great interest has been shown therein.
In the organic EL element, electrons and holes are injected, respectively, from a negative electrode and a positive electrode, and recombined together in the luminescent layer to thereby cause the luminescent layer to emit visible light corresponding to luminescence properties thereof.
The positive electrode is made of a transparent electrical conductive material. Among various transparent electrical conductive materials, indium tin oxide (ITO) is primarily used, in view highest electrical conductivity and relatively large work function, thereby being capable of obtaining high hole-injection efficiency.
On the other hand, the negative electrode is generally made of a metal material. Among various metal electrodes, metal materials such as Mg, Mg/Ag, Mg/In, Al and Li/Al are primarily used, in view of electron-injection efficiency and from the point of work function. Such a metal material has high light reflectance, and therefore takes on a function of reflecting light emitted from the luminescent layer to increase an outgoing light amount (light-emission brightness), in addition to its function as an electrode (negative electrode). That is, light emitted toward the negative electrode is specularly reflected by a surface of the metal material of the negative electrode, and extracted as outgoing light from the transparent ITO electrode (positive electrode).
However, in the organic EL element having such a structure, due to the negative electrode having a mirror surface with strong light reflectivity, outside-light reflection undesirably becomes prominent in a non-luminous state. That is, there is a problem that reflected glare of indoor lighting is so terrific during observation, and it becomes difficult to achieve color expression of black under bright conditions, i.e., contrast under bright conditions is extremely poor for use as a light source for a display device.
As measures for improving this problem, there has been disclosed a technique using a circularly polarizing element as a means to prevent the outside-light reflection by the mirror surface (see, for example, the following Patent Literature 1). In a circularly polarizing element disclosed in the Patent Literature 1, an absorption-type linear polarizer and a retardation film having an in-plane retardation of substantially a quarter-wavelength are laminated in such a manner that optical axes thereof intersect at 45° or 135°.
Assume here that the retardation film having an in-plane retardation of substantially a quarter-wavelength is formed, for example, using one sheet of stretched film. In this case, due to wavelength dispersion in which a refractive index of a resulting stretched film varies with respect to each wavelength, the retardation can become approximately just a quarter-wavelength with respect to a certain wavelength, but it can deviate from a quarter-wavelength with respect to another wavelength. As a result, depending on wavelength, the above retardation film is likely to fail to function as a retardation film having an in-plane retardation of a quarter-wavelength. That is, there is a problem that, when the retardation film is configured to function as a retardation film having an in-plane retardation of a quarter-wavelength with respect to green light of 550 nm wavelength, it becomes difficult to completely prevent reflection of red light having a longer wavelength and blue light having a shorter wavelength, as compared to the green light, and, in particular, a deviation of retardation with respect to blue light is large, resulting in bluish reflected hue.
In this situation, for preventing reflection for all wavelengths of visible light, it is necessary to have a reverse wavelength dispersion property (in which a retardation value becomes larger as the wavelength becomes longer) capable of exhibiting a quarter-wavelength retardation value in the entire wavelength range. As a film capable of exhibiting such a reverse wavelength dispersion property, there have been known films described in the following Patent Literatures 2 to 4.
The Patent Literatures 2 discloses a retardation plate, wherein a specific resin is used to form a single layer having a reverse wavelength dispersion property capable of exhibiting a λ/4 retardation value in the entire wavelength range. The Patent Literatures 2 also discloses that a retardation film obtained by providing a perpendicularly-oriented liquid crystal layer on an obliquely-stretched cellulose acylate film has a quarter-wavelength retardation in a wide wavelength range, and an organic EL display device provided with the retardation film is improved in terms of hue fluctuation due to outside-light reflection. However, an elongated circularly polarizing plate produced by using the retardation film described in the Parent Literature 2 has a problem that, due to degradation of outside-light reflection preventive property, a hue thereof in a non-luminous state is shifted from black. The retardation film described in the Patent Literature 2 is configured to use a component other than the cellulose acylate to develop the retardation and the reverse wavelength dispersion property, which causes a problem of a large variation in the reverse wavelength dispersion property due to stress.
The Patent Literature 3 discloses a retardation film, wherein a cellulose acylate resin containing cellulose ether is used to form a single layer having a reverse wavelength dispersion property. However, the retardation film described in the Parent Literature 3 has a problem that a wavelength dispersion property varies according to changes in humidity environment. This results in a problem that a hue of an obtained circularly polarizing plate undesirably varies. Moreover, the retardation film described in the Patent Literature 3 is configured to use the cellulose acylate resin to develop the reverse wavelength dispersion property, which causes a problem that the hue is more likely to fluctuate in an environment where humidity fluctuates.
The Parent Literature 4 discloses a retardation film, wherein a cellulose acetate resin containing a compound with a specific structure is used to form a single layer having a reverse wavelength dispersion property. However, an elongated circularly polarizing plate produced by using the retardation film described in the Parent Literature 4 has a problem that both the hue variation and the hue fluctuation along with humidity environments occur. Moreover, the retardation film described in the Patent Literature 4 is insufficient in terms of retardation developability, and therefore a film thickness thereof needs to be increased to achieve a quarter-retardation, which caused a problem of an increase in cost and difficulty in thickness reduction of an image display device.